We have defined the conditions required to allow rat parenchymal hepatocytes in primary culture to respond to growth factors present in the rat serum. The effect of partially hepatectomized rat serum is consistently higher than that of control rat serum. We have shown that the activity responsible for the increased growth stimulation by the hepatectomized rat serum is not due to differences in the levels of insulin, EGF, PDGF, hydrocortisone, triiodothyronine, or glucagon. Norepinephrine can stimulate proliferation of hepatocytes through the alpha-1 receptor in the presence of EGF. The concentrations of catecholaminein peripheral plasma are elevated after partial hepatectomy to levels approaching twice those of the sham operated animals. These concentrations in the peripheral plasma are the same as the ones that stimulate hepatocyte proliferation in culture. EGF can stimulate hepatocyte proliferation in culture in the presence of nonessential amino acids or proline by itself. In the absence of the proper amino acid environment the effect of EGF is totally abolished. Using serum fractionation techniques we found that the hepatopoietic activity of the rat serum is due to the synergistic effects of two factors, (hepatopoietins A and B) of large and small molecular weight respectively. We found that hepatocellular carcinoma cell lines produce large molecular weight proteins that stimulate the proliferation of hepatocytes in culture. These proteins mimic the effect of EGF and compete with EGF for binding with the EGF receptor. A possible involvement of catecholamines is suggested but the exact nature of involvement is not clear. The purpose of this project is to: (1)\purify, isolate and characterize the hepatopoietins; (2)\measure these substances to determine their levels during hepatic regeneration and their sources of origin; (3)\determine the growth stimulatory effects of these factors and compare their effects on normal hepatocytes and cells from hepatocellular carcinomas initiated by chemical carcinogens; (4)\examine the possibility that the neoplastic nature of the hepatocellular carcinomas may be dependent on either abnormal response to hepatopoietins or abnormal production by hepatocarcinomas of these factors or factors which mimic the hepatopoietin effect.